Recently, accompanied by rapid spreading of information devices and communication devices, such as personal computers, video cameras and cell phones, development of batteries for power supply thereof has become more important. Furthermore, also in the field of vehicles, development of batteries having high output and high capacity for electric cars and hybrid cars has been promoted. Among several kinds of batteries used therefor, attention is focused on lithium ion secondary batteries because they have high energy density and output. A typical lithium ion secondary battery is formed by a cathode active material layer, an anode active material layer, and an electrolyte between the cathode and anode active material layers.
On the other hand, attention is also focused on air batteries as a secondary battery having high capacity. Japanese Unexamined Patent Application Publication No. 2011-134628 discloses a lithium air battery in which water-soluble electrolytic solution is used at the air electrode side. This lithium air battery consists of an anode, an organic electrolytic solution for the anode, a separator of solid electrolyte, a water-soluble electrolytic solution for an air electrode, and the air electrode, in this order. A material that does not allow penetration of a water component, dissolved gas, protons (H+), hydroxide ions (OH−) or the like, is provided as the solid electrolyte.
Furthermore, the all-solid lithium ion battery is a lithium ion battery in which a solid electrolyte is used as the electrolyte. The all-solid lithium ion battery is a focus of attention as an alternative to commercially available lithium ion secondary batteries in which an organic electrolytic solution is used as the electrolyte, since it has no risk of electrolytic solution leakage and gas generation.
A material having high lithium ion conductivity is necessary as the solid electrolyte for the air battery and the all-solid lithium ion battery. As such material having high lithium ion conductivity, lithium-lanthanum-titanium oxides are the focus of attention (See Japanese Unexamined Patent Application Publications Nos. 2010-262876 and 2011-222415).
The document “Y. Inaguma, et al., Solid State Communications 689-693(1993) 86.” discloses that lithium-lanthanum-titanium oxide is produced by using lanthanum oxide, lithium carbonate and titanium oxide, mixing them by a solid phase method, and performing heat treatment. It also discloses that lithium-lanthanum-titanium oxide produced by this method exhibits high lithium ion conductivity of 7×10−5 Scm−1.
Furthermore, the document “A. Mei, et al., Solid State Ionics 2255-2259 (2008) 179.” discloses that lithium-lanthanum-titanium oxide sintered material having SiO2 concentration of 0.58 to 2.89 mass % is produced by using lithium nitrate, lanthanum nitrate and tetrabutyltitanate as a raw material, mixing them by a liquid phase method, performing heat treatment so as to synthesize lithium-lanthanum-titanium oxide, adding tetraethyl ortho silicate, and performing heat treatment. It also discloses that lithium-lanthanum-titanium oxide sintered material produced by this method exhibits improved lithium ion conductivity up to 8.9×10−5 Scm−1 (SiO2 concentration 2.31 mass %, a measuring temperature of 30° C.).
Furthermore, US Unexamined Patent Application Publication No. 2011/0318650 discloses that lithium-lanthanum-titanium oxide having Al2O3 concentration of 11.1 mass % is produced by using lanthanum oxide, lithium carbonate and titanium oxide, mixing them by a solid phase method, and performing heat treatment so as to synthesize lithium-lanthanum-titanium oxide, adding Al2O3, and performing heat treatment. It also discloses that lithium-lanthanum-titanium oxide sintered material produced by this method exhibits improved lithium ion conductivity up to 9.33×10−4 Scm−1 at the inside of a particle and 2.38×10−5 Scm−1 at an interface between particles (a measuring temperature of 30° C.).
From the viewpoint of increasing output of a battery, a solid electrolyte material having high lithium ion conductivity is required. An object of the present invention is to provide a lithium-lanthanum-titanium oxide sintered material having lithium ion conductivity of 3.0×10−4 Scm−1 or more at a measuring temperature of 27° C. as the solid electrolyte material, and a method for producing the same.